1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a power storage device, a semiconductor device, a display device, a light-emitting device, a driving method thereof, or a fabrication method thereof. In particular, the present invention relates to, for example, an electrode for a storage battery and a method for forming the electrode for a storage battery.
2. Description of the Related Art
With the recent rapid spread of portable electronic devices such as mobile phones, smartphones, electronic book (e-book) readers, and portable game machines, secondary batteries for drive power sources have been increasingly required to be smaller and to have higher capacity. Storage batteries typified by lithium-ion secondary batteries, which have advantages such as high energy density and high capacity, have been widely used as secondary batteries used for portable electronic devices.
Further, there is a significant effect on social living and personal lives when power supply equipment malfunctions or is partly broken and an electric power company stops or restricts power supply because of natural disasters or accidents. For this reason, demand for floor-mounted (stationary) storage batteries that can secure electric energy has grown.
A lithium-ion secondary battery, which is one of storage batteries and widely used because of its high energy density, includes a positive electrode including an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode formed of a carbon material such as graphite capable of occlusion and release of lithium ions, a nonaqueous electrolytic solution in which an electrolyte formed of a lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate (EC) or diethyl carbonate (DEC), for example. The lithium-ion secondary battery is charged and discharged in such a way that lithium ions in the secondary battery move between the positive electrode and the negative electrode through the nonaqueous electrolytic solution and inserted into or extracted from the active materials of the positive electrode and the negative electrode.
In recent years, attention has been focused on a sodium-ion secondary battery because sodium is a naturally abundant element. In the sodium-ion secondary battery, an electrode material including sodium is used, and sodium ions move between a positive electrode and a negative electrode to enable a battery reaction.
Lithium iron phosphate used as a positive electrode active material is also referred to as a polyanionic compound. The polyanionic compound refers to a compound with a structure in which oxyanions are three-dimensionally bonded to each other. Such a polyanionic compound has high cell potential but tends to have low electric conductivity.
In an electrode used in a battery, it is important to form a path for favorable electric conduction from a current collector to an active material on an electrode surface. Low electric conductivity of the electrode impairs cycle performance of the battery or reduces a discharging rate.
In view of the above, to increase electric conductivity between a positive electrode current collector and a positive electrode active material, for example, particles of acetylene black (AB) or graphite (black lead) having higher electric conductivity than the positive electrode active material are mixed as a conductive additive, which increases electric conductivity between particles of the active material or between particles of the active material and the current collector (see Patent Document 1).